Ultraviolet radiation generators are known for coupling microwave energy to an electrodeless lamp, such as an ultraviolet (UV) plasma bulb mounted within a microwave chamber of an ultraviolet lamp system. In ultraviolet lamp drying (heating) and curing applications, one or more magnetrons are typically provided in the lamp system to couple microwave radiation to the plasma bulb mounted within the microwave chamber. The magnetrons are coupled to the microwave chamber through one or more waveguides that include output ports connected to an upper end of the chamber. The microwave chamber has coupling slots or antennas positioned at or near the outlet ports of the waveguides for coupling the microwave radiation to the plasma bulb. When the plasma bulb is sufficiently excited by the microwave energy, it emits ultraviolet radiation through a bottom end of the microwave chamber toward a substrate to be irradiated. While the coupling slots or antennas are capable of coupling the microwave energy into the microwave chamber, they have a known drawback of creating fringe energy fields that form potentially damaging regions of concentrated microwave energy near the ends of the bulb. The fringe energy fields generated in the vicinity of the coupling structures act aggressively with the plasma bulb to cause local heating of the bulb envelope near the ends of the bulb. This localized heating of the bulb envelope generally shortens the bulb's operating life.
Typically, the microwave chamber of the UV lamp system includes a mesh screen mounted to the bottom end of the chamber that is transmissive to ultraviolet radiation but is opaque to microwaves. UV lamp systems used in curing of adhesives, sealants or coatings, for example, typically include a reflector mounted within the microwave chamber that is operable to focus the emitted ultraviolet radiation in a predetermined pattern toward the substrate to be irradiated. The reflector may be metallic and form part of the microwave chamber or, alternatively, may comprise a coated glass reflector mounted within the chamber. It will be appreciated that the terms “upper end” and “bottom end” are used herein to simplify description of the microwave chamber in connection with the orientation of the chamber as shown in the figures. Of course, the orientation of the microwave chamber may change depending on the particular ultraviolet lamp drying (heating) or curing application without altering the structure or function of the microwave chamber in any way.
In UV lamp systems, the efficiency and reliability of the plasma bulb is affected by the uniformity of the microwave field created in the microwave chamber. If regions of the plasma within the bulb are not sufficiently excited by microwave energy, localized areas of minimal ultraviolet radiation may be formed along the longitudinal axis of the plasma bulb, thereby providing a generally non-uniform light output from the plasma bulb. On the other hand, if regions of high local fields are generated in the bulb, such as created by coupling structures formed in the path of the propagating microwave energy, local heating of the bulb envelope may occur that results in shorter bulb life and a reduction in bulb performance and reliability.
Accordingly, there is a need for an ultraviolet radiation generator that couples microwave energy to a plasma bulb in a controlled and efficient manner. There is also a need for an ultraviolet radiation generator that improves the light output uniformity of the plasma bulb along its longitudinal length. There is yet also a need for an ultraviolet radiation generator that improves bulb life by reducing the occurrence of potentially damaging high local fields along the length of the plasma bulb.